1. Field of the Invention
The invention relates in general to protective relays, and more specifically to protective relays of the solid state, overcurrent type.
2. Description of the Prior Art
Overcurrent relays constructed of solid state components require efficient and economical means for providing both an information signal and operating power. A saturable current transformer may be used, with both the current information and circuit power being supplied by this saturable current transformer. The line current information is interpreted from the current transformer's secondary voltage peaks. However, the relationship between the actual line current and these voltage peaks is unique and consistent only when the waveshape and frequency of the line current are defined. If the design is based upon a nominally 60 Hz sinusoidal line current status, the saturated secondary voltage peaks representing the line current value may suffer inaccuracies or become meaningless when the line current contains excessive harmonic and D.C. components. Of course, a linear current transformer may be used for current measurement, and the circuit operating power may be provided either externally or by an auxiliary saturable current transformer. Obtaining power by these methods, however, will complicate the overall relay circuit design and increase its cost.
Co-pending application Ser. No. 648,689, filed Jan. 13, 1976, which is a continuation of application Ser. No. 527,460, filed Nov. 26, 1974, now abandoned, both of which are assigned to the same assignee as the present application, discloses an arrangement which utilizes a linear current transformer having two secondary windings. A solid state switching circuit sequences the two secondary windings to replenish a power supply with the first secondary winding, and then to provide a current signal with the second secondary winding, both during a current half cycle. The primary ampere turns are substantially completely balanced by the second secondary winding when it is providing the information signal, to provide a highly accurate signal.
German Patent No. 1,961,686, dated June 9, 1971, discloses the use of one half wave of a three-phase full-wave bridge rectifier to provide an operating voltage, and the other half wave of the bridge rectifier for a current signal.
It would be desirable to be able to simultaneously provide an information signal and operating power from a single linear current transformer having a single secondary winding. However, it is also desirable that the information signal be highly accurate, tracking changes in the current being measured in a linear manner.